1. Field of the Invention
The present invention relates to a sulfur-containing free-cutting steel used as a material in parts that do not require a great deal of strength, in which SUM steels stipulated by JIS and SAE 11xx and SAE 12xx steels stipulated by SAE standards are utilized.
2. Description of the Related Art
S-containing free-cutting steels, such as JIS SUM steels, SAE 11xx steels or SAE 12xx steels are drawn after being rolled, and are used in automatic machining as polished rod steels. Sulfur-containing free-cutting steels in which S is added to the steel in order to improve the machinability of the steel by means of high-speed steel tools have been used as conventional free-cutting steels of this type.
The machinability of such sulfur-containing free-cutting steels improve with an increase in the S content; on the other hand, however, defective products suffering from cracking and the like are generated in large quantities because of red-shortness during hot working such as rolling, forging and the like. This is caused by the precipitation of low-melting-point FeS in the grain boundaries due to the high sulfur content. Furthermore, in the case of high-S steels, the ductility and reduction of area in the lateral direction with respect to the direction of rolling drop, so that trouble occurs during drawing. Accordingly, 0.35% has generally been set as the upper limit of the S content, and at the very most, this content has been limited to 0.40%.
Furthermore, composite free-cutting steels which contain heavy metals such as Pb, Te, Bi or the like in addition to S have been developed as free-cutting steels that have superior machinability. In recent years, however, environmental problems have been viewed with increasing seriousness, so that there has been a demand for the development of free-cutting steels which do not use such heavy metals that have a detrimental effect on the environment, and which have a good machinability comparable to or superior to that of free-cutting steels that contain heavy metals.
It is an object of the present invention to provide a sulfur-containing free-cutting steel with superior machinability, which does not achieve improved machinability by the addition of heavy metals that have a detrimental effect on the environment, and which does not cause problems during manufacture, especially during hot working or cold drawing.
The present invention is a high-sulfur free-cutting steel which has a chemical composition comprising, in mass %, 0.03 to 0.20% C, 0.35% or less Si (including 0%), 0.30 to 2.00% Mn, 0.01 to 0.15% P, 0.35 to 0.65% S, 0.0100 to 0.0250% O, 0.020% or less N, 0.005% or less Al (including 0%), 0.02 to 0.20% Nb, and further containing 0.05 to 0.50% V or 0.02 to 0.20% Ti, or both, with the remainder consisting of Fe and unavoidable impurities, wherein sulfide type inclusions as principal nonmetallic inclusions contained in the steel have a mean size of 50 xcexcm2 or less and are present at the rate of 500 to 1000 inclusions per mm2 in the cross section of the steel.
First of all, in the present invention, the S content is a large S content that exceeds the 0.35% conventionally considered, to be the upper limit. In order to prevent the occurrence of deleterious effects such as hot brittleness and the like caused by such a large S content, the precipitation of FeS is prevented by including a large quantity of Mn, so that only MnS type sulfides are precipitated.
Furthermore, it was discovered that good free-cutting properties can be obtained by increasing the frequency of contact between these MnS type sulfides and the cutting tool.
Accordingly, although the precipitation of MnS type sulfides into the steel begins from the time of solidification of the molten steel, it was discovered that the inclusions can be made finer by utilizing TiN, which precipitates into the molten steel at the temperature of the molten steel, and NbN and VN, which precipitate into the xcex3-iron during the solidification process, as nuclei for the precipitation of MnS type sulfides, so that the number of precipitated inclusions is increased; furthermore, it was discovered that a uniform dispersion of these inclusions can be obtained.
Accordingly, in order to eliminate the presence of xcex1-type Al2O3 inclusions that shorten the tool life, the joint deoxidation of Sixe2x80x94Mn was used as a base for deoxidation of the molten steel instead of using Al. Furthermore, hard silicate type oxide inclusions were minimized by lowering the Si content to 0.35% or less, and V or Ti, or both, were added in addition to Nb as deoxidation assistants in order to maintain the oxygen level in the molten steel following deoxidation at a stable 0.01 to 0.025%. It was discovered that MnS type sulfides can be finely and uniformly dispersed and precipitated by utilizing the residues of these elements in the molten steel as nuclei for the precipitation of such MnS type sulfides. The residues referred to here also naturally include oxides of Nb and the like; it appears entirely possible that these substances also serve as bonding agents in the form of composite inclusions and nuclei for the precipitation of MnS type inclusions.
Furthermore, it was discovered that as a result of the oxygen level being maintained at 0.01 to 0.0250%, the hardness of the precipitated MnS type sulfides also drops, thus prolonging the tool life and reducing the aspect ratio of MnS inclusion (ratio of the length to the diameter of the MnS inclusion), so that the chip breakability is improved.
The three discoveries mentioned above form the basis of the present invention. A sulfur-containing free-cutting steel was developed which has workability well comparable to or superior to that of steels containing heavy metals such as Pb, Bi, Te and the like, without requiring the addition of such heavy metals.